Collins Chinese Dictionary Plus (3rd edition), 2011:
未知数(數) wèizhīshù 名
1 (数) unknown number
2
(=未知事物)
- 他能否成名还(還)是个(個)未知数(數)。 Tā néngfǒu chéngmíng hái shì gè wèizhīshù. It is uncertain whether he will become famous or not.